Make alt text limit an .env parameter

[sneakers-the-rat]

Alt text should be able to be one million characters long. This pull requests lets the alt text limit be set from .env as MAX_ALT_TEXT_CHARS (defaulting to the current 1500.

Also added to /api/instance and /api/v1/instance

Alt text config implementation:

• Set in .env (with example in .env.production.sample)
• Set in Rails in MediaAttachment model as MAX_DESCRIPTION_LENGTH by reading from ENV
• Read by Rails in initial_state_serializer and instance_serializer for both v1 and v2 apis
• Transferred to js from initialState as maxAltTextChars
• Used in focal_point_modal to display.

API Changes:

• /api/v1/instance
  • max_alt_text_characters
  • configuration.media_attachments.max_alt_text_characters
• /api/instance
  • configuration.media_attachments.max_alt_text_characters

Also fixed an unexplained reversion in https://github.com/mastodon/mastodon/commit/71f2b95106b2e75d3efb40040b29c216c2d99ee6 that invalidates alt text that is longer than the local limit.

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.