Tip label can be defined in display_defaults

[jameshadfield]

The ability to encode the tip-label in the URL (?tl=...) was implemented at the same time as the ability to change tip-label keys was done - see 0e55b30

The ability to encode this in the JSON's display_defaults wasn't done, presumably due to time constraints.

This commit was originally part of the larger #1668, however that's had no comments on it and so I don't know if/when it'll be merged.

Augur PR nextstrain/augur#1298 will update the schema to allow this field

[corneliusroemer]

Let's merge this soon as the schema already advertises tip labels as settable because https://github.com/nextstrain/augur/pull/1299/files was merged. I didn't check if there was already a PR and redid the work in #1703

[corneliusroemer]

Tested an it works - though I don't know all the details of why the middleware changes are necessary, in my PR I seemed to be able to get away without.

This one has much better validation and documentation.

Tree to test on: nextclade_mpox_b1.json.txt

[jameshadfield]

This should have been merged & released before nextstrain/augur@6ce1bf7 but that's not the way it played out. Thanks for the reminder @corneliusroemer!

though I don't know all the details of why the middleware changes are necessary, in my PR I seemed to be able to get away without.

The middleware (in this context) keeps the URL query tl=... up-to-date - if the selected tip label matches the JSON-set default then we don't want any URL query, as the URL query is intended to change state from the defaults.

[jameshadfield]

[Rebased onto master]