Move Create Gallery button and make it contextual

[mor10]

Currently the Create Gallery button appears in the left-hand sidebar in the Media Modal at all times causing unexpected behavior when multiple images are selected and then have to be re-selected before the gallery is created.

Proposal:
(part of ImageFlow project, full rundown here: ​https://github.com/ImageFlow/imageflow-plugin/blob/master/Issues/gallery.md)

1. Move Create Gallery button from upper left corner to lower right corner next to Insert Into Post button
2. Make Create Gallery button appearance contingent on multiple image selection. If only one image is uploaded/selected the Create Gallery button should be greyed out.
3. On click the Create Gallery button should take the user directly to the Edit Gallery Options panel where order, columns, link to, etc is configured.

[mor10]

"Create Gallery" link removed from left sidebar and "Create Gallery" button added to lower right corner conditionally.

[jorbin]

While this appears to be a UX issue, it's not a bug in the code (which is functioning as expected). Therefore, reclassifying.

[karmatosed]

With Gutenberg having a gallery or image insertion, I wonder if this is something to consider anymore. For now as they are being split there, I think marking this as being part of that. Its not contextual, but it is a flow that avoids the current problem.