Resolves #2440 Image paste from clipboard support

[effone]

Resolves #2440

Image paste from clipboard support added.

[dvz]

Do we want inline image support in the long run (rather than handling them as attachments client-side when pasted/dropped)? Best to go over this before adding support for formats that may be dropped later @mybb/developers

[euantorano]

IMO this is what should happen:

1. Image pasted/dropped in to editor
2. JS detects this, and starts an AJAX request to create an attachment
3. If attachment added, atatchment is inserted in post/thread/PM (note: PMs don't support attachments in 1.8...)

In the long run, base 64 content for what could be large (several MB) image files inside the posts table would only cause headaches.

However, implementing the above wouldn't be finished by 1.8.22 unless we want to delay the release by several weeks.

I wouldn't mind stripping out the base 64 support for now and creating a new issue for 1.8.23 to implement the above flow. Or we can leave it as is for 1.8.22 and implement the above for 1.8.23 - only problem will be any posts created between 1.8.22 and 1.8.23 could have inline base 64 image data.

[dvz]

Yes, we could revert this one to avoid potential backtracking on MyCode support (individual forums could still install a plugin for this, which should be simple to create).

[euantorano]

I think that would be for the best for now.