Agnostic media modal

• Allows media modal to display more than just activity entities. Now can take objects (image/video/blog) too, so it can be used for Pro
• Display comment attachments in media modal

Closes #1822 (closed)

TESTING:

Test at rubbersoul . No need to do Step 1 anymore because there's already content on the test domain

To get to the testing site: click the 'view app' button on this MR page. If there isn't one, click the 4th icon on the pipeline (the row of primarily green checkmarks. The 4th one should be 'review:'). From the dropdown that appears, select 'review:start'

### Step 1: Do a bunch of media-related stuff with a test user account * [ ] Make some posts that have image and video attachments (videos might not work if Mark hasn't enabled them in the staging environment yet, so you might not be able to test videos) * [ ] In the comment sections of those posts, add comments that have image and video attachments * [ ] Some posts/comments should be accompanied with text titles/descriptions, others should just be a single media attachment without any words * [ ] Remind one of the posts from the newsfeed * [ ] Remind one of the posts from its media page * [ ] Make a group and make an image/video comment in group chat

Step 2: Try to break it

• Go through posts on a desktop and make sure they open the modal when you click on them
• Make sure the modal opens when you click on image/video comments from:
  • The comments section of a post in your newsfeed
  • The comments section of an open modal
  • The comments section of a media page
  • The comments section of a group chat
  • Any other kind of comments section that isn't listed here
• Click the action buttons (wire/thumbs/remind)
• Do anything that isn't included in this list that I haven't thought of
• Do everything in this 'try to break it' section in a variety of browsers (chrome/brave/firefox/safari)
• If you have access to a tablet, try that out too