Upgrade ancestry format and ensure indexed

[wilson29thid]

There's a new format for the ancestry column which is more performant. Also, there's guidance on indexing that we'll want to make sure we follow. This column is used a lot so should make a noticeable difference.

https://github.com/stefankroes/ancestry

• [x] Change Regt HQ's path to / and verify this doesn't break v2
• [x] Make path non-nullable
• [x] Investigate using different collation (utf8mb4_bin) on column, and impact of changing it for existing values
• [x] Add ancestry initializer to upgrade serialisation format

If we want to use the path column:

• [x] Add index to path column
• [ ] Update v3 manage/units pages to use path
• [ ] Drop ancestry column and its index

Or if we want to continue calling it ancestry:

• [ ] Drop ancestry column and its index
• [ ] Rename path to ancestry
• [ ] Add index to new ancestry column
• [ ] Update v2 references from path to ancestry (there are a ton of these, and it looks like a real pain!)

Finally:

• [ ] Remove Unit callback (and any others) that update the path when ancestry is changed
• [ ] Update unit factory to use new format

[wilson29thid]

It looks like the paths are all up-to-date already, thanks to the callback we've had in place in the Unit model. There are 3 root-level units, one of which has a NULL path (Regt HQ), and 13 TPs that have NULL ancestry for some reason.

select id, abbr, path, ancestry
FROM units
where path != concat('/', ancestry, '/')
    or path is null or ancestry is null

id	abbr	path	ancestry
595	Admins	/
624	SLT	/
1631	Regt. HQ
1634	1522nd TP	/1631/361/362/486/43/
1635	1523rd TP	/1631/361/362/486/43/
1636	1524th TP	/1631/361/362/486/43/
1645	1525th TP	/1631/361/362/486/43/
1646	1526th TP	/1631/361/362/486/43/
1647	1527th TP	/1631/361/362/486/43/
1648	1528th TP	/1631/361/362/486/43/
1649	1529th TP	/1631/361/362/486/43/
1650	1530th TP	/1631/361/362/486/43/
1651	1531st TP	/1631/361/362/486/43/
1652	1532nd TP	/1631/361/362/486/43/
1653	1533rd TP	/1631/361/362/486/43/
1654	1534th TP	/1631/361/362/486/43/

[wilson29thid]

So we can't use the column name "path" with the ancestry gem because ancestry already uses that term as a concept.