Feature/1628 muli image select

No ready for merge!

Phew! Almost there. I'm opening this PR a bit early in hopes of gathering feedback from other devs. This project replaces the Post editor with a new tabbed Activity for multiple selection of media content. I believe it reaches feature parity with the existing implementation, so no loss of functionality.

I've got a stash with an effort to replace the Media page as well. I believe doing so will vastly simplify the implementation, we should be able to fully remove:
<ul>
<li>MediaGridFragment</li>
<li>MediaGridAdapter</li>
<li>MediaBroswerActivity</li>
<li>along with a bunch of resources...</li>
</ul>

### Known issues:
<ul>
<li>Animation when entering/exiting ActionMode (flashes a bit)</li>
<li>Syncing thumbnails with images</li>
<li>Keyboard doesn't always disappear when entering the selection Activity</li>
<li>Landscape layout</li>
<li>Every now and then artifacts will momentarily display on the edges of the screen when exiting the Activity via the Back button</li>
<li>Content capture flow needs work all-around (in-progress)</li>
</ul>

### Future features:
<ul>
<li>Replace Media page when Search/Delete functionality implemented</li>
<li>Camera preview on capture items</li>
<li>Multiple content type selection</li>
</ul>

I'll be working through the known issues in the coming days. Progress should be much quicker now that I'm not employed, I'll have much more time to devote to this project and expect to see most of the kinks worked out within the week.