Move Xhr requests to Rest endpoints
## Description
In addition to the Model/Gateway refactoring &1 it would be good to move network requests from the old XHR to new REST endpoints and have a more simple API. There are already several ones in https://gitlab.com/foodsharing-dev/foodsharing/-/tree/master/src/Controller that can be used or extended.
## Goal
* Separate front-end and back-end stuff as there are still many XHR functions that return Javascript or HTML
* Make it easier to work on the UI, e.g. to refactor it into Vue components
* Maybe fix some encoding bugs in the same time
## List of old endpoints
These are all I could find for now, please add more if you find any:
* [x] [ActivityXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Activity/ActivityXhr.php) (still needs refactoring in foodsharing#9)
* [x] [ApplicationXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Application/ApplicationXhr.php)
* [x] [BasketXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Basket/BasketXhr.php)
* [x] [BellXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Bell/BellXhr.php)
* [x] [BuddyXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Buddy/BuddyXhr.php) foodsharing#847 foodsharing!1646
* [x] [EmailXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Email/EmailXhr.php) foodsharing!1354
* [x] [EventXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Event/EventXhr.php)
* [x] [FoodsaverXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Foodsaver/FoodsaverXhr.php)
* [x] [LoginXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Login/LoginXhr.php) foodsharing#819 foodsharing!1632 foodsharing!1642
* [x] [MailboxXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Mailbox/MailboxXhr.php) See: foodsharing#789
* [x] [MainXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Main/MainXhr.php)
* [x] [MapXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Map/MapXhr.php) (standardise the markers to GPX or GeoJSON ?)
* [x] [MessageXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Message/MessageXhr.php) (included in foodsharing!887)
* [x] [ProfileXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Profile/ProfileXhr.php) foodsharing#903
* [x] [QuizXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Quiz/QuizXhr.php)
* [x] [RegionAdminXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/RegionAdmin/RegionAdminXhr.php) foodsharing!1547
* [x] [RegionXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Region/RegionXhr.php)
* [x] [ReportXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Report/ReportXhr.php)
* [x] [SearchXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Search/SearchXhr.php) foodsharing!1522
* [x] [SettingsXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Settings/SettingsXhr.php)
* [x] [StoreXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Store/StoreXhr.php) (still needs refactoring in foodsharing#9, foodsharing!1196) foodsharing#885
* [x] [TeamXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/Team/TeamXhr.php)
* [x] [WallPost](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/WallPost/WallPostXhr.php)
* [x] [WorkGroupXhr](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Modules/WorkGroup/WorkGroupXhr.php)
* [x] [XhrMethods](https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Lib/Xhr/XhrMethods.php) foodsharing#261 foodsharing#823
### Endpoints sorted by extent
```
Modules % find . -name '*Xhr.php' | xargs wc -l | sort -nr
2178 insgesamt
1169 ./Quiz/QuizXhr.php
216 ./Basket/BasketXhr.php
198 ./WorkGroup/WorkGroupXhr.php
129 ./Store/StoreXhr.php
119 ./Report/ReportXhr.php
98 ./Settings/SettingsXhr.php
94 ./Foodsaver/FoodsaverXhr.php
78 ./Team/TeamXhr.php
47 ./Region/RegionXhr.php
30 ./Map/MapXhr.php
```
When working on replacing one specific method, best create an issue for that task and link it with your merge request :)
epic