add `ent tag ISSUE [[-]TAG]`

# Usage

`ent tag ISSUE`: List tags on ISSUE.

`ent tag ISSUE TAG`: Add TAG to ISSUE.

`ent tag ISSUE -TAG`: Remove TAG from ISSUE.


# Notes

Tags are short text strings without white space, and not beginning with
"-".

The Issue directory will gain a file named `tags`, containing the sorted
list of tags.

The `ent list` output will add a bit to the end of each issue displayed,
something like "[tag1, tag2]", or nothing if the issue has no tags
(like how we don't show assignee if it is None).

As part of this issue, the `ent list FILTER` will gain a new filter
chunk type like "tags=v2.31,ux-papercut".