Support setting tags on songs imported from CSV

[jefft]

Fixes #1262. Adds support for setting song tags from CSV, via new column(s).

Code notes

In the code, I needed to create Service_Component_Tags and Service_Component_Taggings only if they didn't already exist. So I copied the createIfNew pattern I found in Person_Note.

createIfNew() is a candidate for being moved up to DB_Object. For that to work, there would need to be a way for each object to tell it "which of my columns are unique". Possibly we could use _getUniqueKeys() (currently only used for FK creation). Anyway, this is a bigger refactoring I didn't tackle.

Testing

Sample file

For basic testing, go to Services -> Component Library -> Import, and download the Sample file:

The imported songs are now tagged:

Detailed testing

This song CSV illustrates setting 0, 1 or 2 tags via multiple columns:

Title,Tag 1,Tag 2
Tagged Test Song 1,TagOne,
Untagged Test Song 2,,
Tagged Test Song 3,TagTwo,
Tagged Test Song 4,TagOne,TagTwo

Imported songs are tagged as expected: