Entity Export Dropdown on Index

Fixes

Fixes for #786 (closed) by @shivohamangal

Description

This pr adds a dropdown to the index page of entities so that data can be downloaded in - txt, csv, json, ttl and json-rdf.

Currently, exports for the following entities are fully functional -

1. Archives
2. ArtifactTypes
3. Collections
4. Dynasties
5. Genres
6. Languages
7. MaterialAspects
8. MaterialColors
9. Materials
10. Proveniences
11. Regions
12. Rulers

The following don't have the getCidocCrm() method (maybe @larsgw can implement them?), hence flat data export works, linked doesn't -

1. Abbreviations
2. Aut‎hors‎
3. ExternalResources
4. Jour‎nals‎
5. SignRe‎adings‎

These remaining entities have other issues -

1. Periods: ttl: blank-white-page, rdf: [], table export: serialization failed(yes, data is serialized in the controller)

Alternatively, view the second page of this doc for some color-coded statuses :D https://docs.google.com/document/d/1zdF4bN2V6hwf7pKM2g4EPb7ouqf4eIbam8P2bcWuGL0/edit?usp=sharing

Type of PR This PR is an improvement.

Technicalities

Tests

1. Goto http://127.0.0.1:2354/collections
2. Goto http://127.0.0.1:2354/abbreviations
3. Similarly, go to all the entities mentioned in the list above
   On all these pages, you can see the dropdown. Screenshots

Checklist:

• My pull request has a descriptive title (not a vague title like "Update index.md").
• My pull request targets the phoenix/develop branch of the repository.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.