Tag edit features broken on mobile

[alethiophile]

LRR Version and OS
dev branch, Linux

Bug Details
When using a mobile browser, you can't use a plugin to import metadata. Instead, an error appears talking about Edit.tagInput.

In edit.js, the tagger instance assigned to Edit.tagInput is not initialized if the user is on a mobile browser. However, later code still depends on this object. In particular, the getTags code that calls a server-side plugin unconditionally uses tagInput.add_tag to add the new tags to the field. This fails on mobile.

I assume that pasting tags into the field is also broken, since it depends on the same function. The obvious fix is simply to detect whether the tagInput object is initialized, and if not then edit the textarea directly.