Bugfix in tag creations in the Russian language

[A-Mikhail]

In Russian keyboard layout letter "б" work as a comma, that in the library used to separate tags between each other as well as tab and enter.

The bug prevents writing any words containing this letter.

[jwhitlock]

@A-Mikhail Thanks for tackling bug 1298017.

Unfortunately, this PR is a bit of a mess. It looks like your last commit did a global search and replace for "let" to "var", changing variable declarations but also "deleted" to "devared" and "autocomplete" to "autocompvare", which probably breaks it.

Our copy of tag-it.js was added in dc657b5, from the version at aehlke/tag-it@f5fe54e. The tag-it maintainers continued making changes, but didn't bump the version number (this was a few changes after v2.0 was tagged).

This PR resembles the latest code from https://github.com/aehlke/tag-it, but not quite, and with a patch similar to aehlke/tag-it#251 applied. It would be clearer if this was explicit in the commits, breaking the change into a few commits:

1. A commit from the current version in Kuma to the latest in aehlke/tag-it. It would be nice to identify the current tag-it commit in the comments, since the upstream project does a poor job of bumping version numbers.
2. A commit fixing any linting errors, to make it clear where we're deviating from upstream.
3. A commit bringing back changes from 6280ffb#diff-40e89d0482b1bd7797dd5f7ceecd748f and de10cdb#diff-40e89d0482b1bd7797dd5f7ceecd748f, if needed, or adjusted to work with new tag-it.js features.
4. A commit fixing the issue with "б"/"," , which probably requires switching from a keyDown to keyPress handler.

I wonder if there is a more recent, maintained library that would be better for entering tags (I couldn't see a good one linked from the django-taggit docs, or if the goal of "pretty" tags is a bad one, and we should just have a plain, comma-delimited text field.

[jwhitlock]

There's still a lot of work to make this PR mergable. Let's revisit after some sidebar work in Q1 2019 - I suspect tags will be a bit different after that project.