Media library improvements

[erquhart]

The media library is about to be released - this is just a quick, public place for me to stash improvements prior to release. Most, if not all, of these will be converted to standalone issues.

• double-click on an asset should insert the asset (when insertion is available)
• display count of loaded assets in the UI

[erquhart]

Need to split this out into two separate issues.

[t1merickson]

Do we yet have an issue for making the media library a proper page instead of just a modal?

[erquhart]

@neutyp no, we still need that.

Update: see #962.

[erquhart]

These other bits here aren't too big, the double click bit was more applicable when we were using a table. Closing.