I spotted this by inspection: adding a media time and wall time didn't
look right. I also confirmed the brokenness on my primary NVR:
```
sqlite> .mode column
sqlite> select
...> r1.composite_id,
...> r1.prev_media_duration_90k,
...> r1.wall_duration_90k,
...> r1.media_duration_delta_90k,
...> r2.composite_id,
...> r2.prev_media_duration_90k
...> from
...> recording r1 join recording r2 on (r1.composite_id = r2.composite_id - 1)
...> where
...> r1.prev_media_duration_90k + r1.wall_duration_90k + r1.media_duration_delta_90k !=
...> r2.prev_media_duration_90k
...> limit 5;
4296791095 2232623913716 5398956 154 4296791096 2232629312672
4296791096 2232629312672 5400016 38 4296791097 2232634712688
4296791097 2232634712688 5400729 105 4296791098 2232640113417
4296791098 2232640113417 5399024 80 4296791099 2232645512441
4296791099 2232645512441 5400770 124 4296791100 2232650913211
```
In the first row, the second recording's prev_media_duration_90k is the
first's prev_media_duration_90k plus its wall time, not its media time.
I also enforced some invariants in the signals code, fixing a couple
bugs. The signals code is more complex than I'd like, but hopefully
is working now.
I'm tired of all the boilerplate, so use the new
GPL-3.0-linking-exception license identifier instead in all the server
components.
I left the ui stuff alone because I'm just going to replace it (#111).
Add a checker for the header because it's easy to forget.
Besides being more clear about what belongs to which, this helps with
docker caching. The server and ui parts are only rebuilt when their
respective subdirectories change.
Extend this a bit further by making the webpack build not depend on
the target architecture. And adding cache dirs so parts of the server
and ui build process can be reused when layer-wide caching fails.
