Stack compose form warnings and add a sensitive attachment hint to the compose form

[letydoesstuff]

This PR addresses both #29952 and #29953.

The compose form warning container is rewritten to order the various warnings in the following order of importance:

1. Sensitive media hint explaining new compose form functionality
2. Hashtag warning about reduced hashtag visibility on non-public posts
3. Publicity-related disclaimers and warnings

These warnings are now non-exclusive, meaning they render in a stack with all applicable warnings appearing. Previous behavior was that publicity-related disclaimers and warnings overwrote the hashtag warning.

Additionally, a sensitive attachment hint has been added to explain the "Add content warning" button's new functionality. This hint only appears if the post is marked with a content warning and the post has an attachment added.

(Please ignore the 10k character limit in the compose form. That's exclusive to my fork and entirely excluded from this PR.)

[ClearlyClaire]

Thanks for your contribution! Looks good overall. I'll ask the design team for review around the design changes.