expand homoglyph protection to UserProfile.display_name and Collection.name

[eviljeff]

Fixes: mozilla/addons#15875

Description

Add homoglyph expansion to DeniedName checks used by Collection name, slug, and UserProfile display name. Needed some reworking of how matches to DeniedNames worked now we aren't doing a single check per field.

Context

Prior to the second commit, DeniedName.blocked executed a query each time it was called so it really (really) did not scale up to 65k iterations with homoglyphs. (A single test for l * 16 was taking 90s locally). The python x in y string test is not as loose as the mysql query - the collation used specifically ignored accent characters and case - but my thinking was the homoglyph expansion would effectively do the same thing. (... not a completely proven hypothesis ...)

Testing

• add some DeniedName instances in django admin (if you use django shell, you need to be careful to use lower case)
• make a change to your display name via frontend that contains one of the denied names - see it's blocked
• add a string with 16+ possible homoglyphs - e.g. l more than 16 times.
• (change it to something innocuous to check for regressions)
• repeat with a collection name and slug
• (collections should allow the DeniedName check to be bypassed when you have Collections:Edit; collection slugs have further restrictions on allowed characters)

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.