Move all database access to IndexRepository

[haszi]

Move all database access to IndexRepository.
Use the same database during the entire indexing-rendering process.
Add a simple smoke test for render.php.

This PR is the same as #120 with the following exceptions:

• functions.php's requireIndexing() is now checking whether Config's indexcache exists (where the indexing database used everywhere is stored in and after this PR), therefore the SQLite database parameter has been removed
• indexcache is set for every run so setting it after indexing has been removed in render.php
• added the $config parameter (of type Config) to make_reader() in render.php, calling it with new Config arguments at call sites (this was the issue that broke the builds in the last PR, thanks to @sy-records for pointing it out)
• added a smoke test for render.php so that the last PR's issues can be caught immediately in a local dev environment too

[Girgias]

Hopefully this works this time :D