read/write F-Droid files always as UTF-8

read/write F-Droid files always as UTF-8

This makes UTF-8 the sole supported encoding for F-Droid's files. This is
mostly codifying the already existing practice for config.py and index.xml.
The other files where always just ASCII before.

* config.py
* metadata/*.txt
* known_apks.txt
* categories.txt
* latestapps.txt
* latestapps.dat
* index.xml

Note: this does not change the read/write encoding of stats files.  That is
still ASCII.