Add and use new RemarkupMetadata class

Summary:
Add a new RemarkupMetadata class and use with upload button as well as drag and drop pathways.

With this change, files are now attached automatically to the object. This this could solve
several issues, for example when a person uploads a file but only that author is able to see it.

Ref T15106

Test Plan:

• Drag and drop file, upload file with button. Check that both files are attached with "attachedFilePHIDs" values.
  • drop file in a Task description (now works)
  • drop file in a Task comment (now works)
  • drop file in an edited Task comment (still not supported)
• This was already tested in Wikimedia Foundation: https://phabricator.wikimedia.org/D1203

Reviewers: O1 Blessed Committers, avivey, Matthew

Reviewed By: O1 Blessed Committers, avivey, Matthew

Subscribers: MCPCN, avivey, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Tags: #remarkup

Maniphest Tasks: T15106

Differential Revision: https://we.phorge.it/D25052