Add an auto-completer for editor references

[vkbo]

Summary:

This PR adds a custom MetaCompleter class, a context menu that is triggered on lines starting with the @ character. It has the following features:

• On typing @, a list of keyword options appear. The list is shortened when more characters are typed. Typing @c will list @char and @custom. Hitting enter on an option will insert it and a :.
• Typing anything after a keyword, like a space, will open a list of up to 15 tags in that category. Any further typing will look for the typed characters in the list and filter it down. The lookup needs to be consecutive characters, but it's not case sensitive. So "an" will match "Jane" and "Jannet", but not "John".
• The auto-completer will treat multiple references separated by a comma. It uses the same evaluation function as the syntax highlighter for the parsing.

Related Issue(s):

Closes #823

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed