Add search for issues with patches that touch certain file types (javascript, css, ...) or files

Updated: Comment #N

Problem/Motivation

One specific use case:
@nod_ would like the javascript tag added to every issue that touches a js file.

This can be useful in general by being able to search for any issue with a patch that touches say:
css
xml
etc.

Proposed resolution

a) check patches that are uploaded and add tag that matches the file type (like adding javascript tag to issues when a patch includes changes to .js files)

b) add a separate (non-tag) field that gets updated with what file types get touched, and search on that field on the issue.

c) don't track the data, but have a search that searches through all issues, all patches
(this seems... computationally intensive and not a good idea?)

Remaining tasks

discuss the costs of tracking this, vs the benefits to people wanting to be able to locate issues based on the file types patches touch

User interface changes

Another field on the advanced search pages? (Or, none, if uses tags)

API changes

?

vaguely related to #2013222: Add "Issue tasks" to project issues and correlate tasks with handbook documentation.