When repository feature is disabled, show explicit message instead of blank project page
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### Summary
The UX is not great
### Steps to reproduce
1. Create test project
2. Toggle off the `Repository` feature on `/edit#js-general-project-settings`
3. Check project's main page.
### Example Project (instead of logs and/or screenshots)
https://gitlab.com/katrinleinweber_ultimate_group/gl-434367-repo-disabled
### What is the current *bug* behavior?
If the user has set [Project overview content](https://gitlab.com/-/profile/preferences) to `Files and Readme (default)`, the resulting project's main page is a bare-bone version of `/activity` without any indication of that fact or of the disabled feature.
When setting `Activity` instead, the content is more clearly that, but without the `/activity` URL.
### What is the expected *correct* behavior?
Better ~UX, for example an alert/banner and links to `/edit#js-general-project-settings` and mentions the `Repository` toggle.
### Output of checks
This bug happens on GitLab.com
#### ~~Results of GitLab environment info~~
#### ~~Results of GitLab application Check~~
### Possible fixes
issue