Misc fixes

[CBroz1]

Description

• Fixes get_djuser_name error specificity #1175: Adds suggestion of fix to common_lab.py
• Fixes [Export] REGEXP restriction fails on empty list #1183, common_usage.py: Only add external tables if there are files associated with the export
• Fixes DLCModelSource uses fetch1 when restricting by a secondary key #1105: Modifies position_dlc_model.DLCModelSource.insert_entry to prompt user if there are
• Fixes [Export] fetch_nwb assumes fetching table inherits the mixin #1185: dj_helper_fn.fetch_nwb Check that query expression (a) is an instanced table, and (b) inherits the Mixin before setting log_export arg
• WIP: Fixes SpyglassMixinPart rejects restrictions of self on delete #1163: dj_mixin - conditionally applies the restriction to self or master

Checklist:

• No. This PR should be accompanied by a release: (yes/no/unsure)
• N/a If release, I have updated the CITATION.cff
• No. This PR makes edits to table definitions: (yes/no)
• N/a If table edits, I have included an alter snippet for release notes.
• N/a. If this PR makes changes to position, I ran the relevant tests locally.
• I have updated the CHANGELOG.md with PR number and description.
• N/a. I have added/edited docs/notebooks to reflect the changes

[edeno]

Still seeing some weird testing errors