Additional case handling for color parameter in search API

[chrstinalin]

Fixes: mozilla/addons#15022

Description

Handles cases as follows --

1. The empty color parameter is ignored.
2. Otherwise, take the first 6 characters of the color parameter (after removing the #, if any). 3-character hex codes (ex. #aaa) still pass.
3. Throw ValueError if the value is still unparseable as a hex code.

Testing

Directly use the search function for static themes, or to the API directly, ex.
http://olympia.test/api/v5/addons/search/?type=statictheme&color=336699aa

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.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: src/olympia/search/filters.py

Function	Unhandled Issue
get_es_query	TypeError: hsl() takes 3 positional arguments but 4 were given /api/v5/... Event Count: 5
get_es_query	TypeError: hsl() missing 3 required positional arguments: 'r', 'g', and 'b' /... Event Count: 1

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