new migration prototype

- change hook from `init` to `admin_init` → drastically decreases
chances of multiple migrations running at the same time (bonus: one
less option-query in frontend)
- multisites must be handled explicitly now, otherwise the admin has to
visit all of them — and in that time the frontend might be failing
rework support page
system report: add podlove cache state
system report: list assets
fix escaping in database logger

fixes #453
`mysql_real_escape_string()` => `esc_sql()`

Prefer WordPress method to native mysql escaping to be future proof
(hopefully).
fix feed validator: when "pretty permalinks" off

URLs of the form `domain.tld?foo?redirect=no` would be created, which
are invalid. I also refactored the URL generation into a separate
function to dry up the source.
merge
Merge branch 'migration-overhaul' into release-1-10-17
update readme
Merge branch 'release-1-10-16' into release-1-10-17
fix dashboard box state saving
improve CSS for "at a glance" in sidebar
remove "Critical Podlove Warnings"
add repair functionality to support page
wrap translation strings in translation functions
fix: file URLs w/ tracking on but pretty links off
cache open graph html output
update readme
fix migration permission issue