tracker ui: For accessibility, use light background since links are dark.

Also shift text shadow to give effect of source lighting coming from
top-left.
tracker.model: Add some nullable=False.
tracker.form: Change method field to RadioField
services: Add stub code for chain sequences.
tracker: Validate URL template as a prefix.
Update readme about the structure of the code.
tracker: Add export testing
tracker: Add a backup dump script.
tracker: Rename chain.py->rand.py
tracker.service: Add HashRandMixin
Add WIP service tester.
Add isgd test definition

From
https://github.com/ArchiveTeam/tinyback/blob/12d8610b6cbfb49ba4ce060834a14755e255ffcf
/test-definitions/isgd.txt
tracker: Fix up live_test.py
tracker.model: Use [403, 420, 429] as default banned codes.
tracker: Fix up export_test.py
tracker.model: Make min versions not null.
tracker: Fix unit test fail on password checking
tracker: Support hard coded min versions for major api changes.
Clarify the min versions into "library" and "pipeline" versions
client: Support sending user-agent to tracker.