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.

fdroidserver/rewritemeta.py

@@ -33,7 +33,7 @@ def proper_format(app):
     s = io.StringIO()
     # TODO: currently reading entire file again, should reuse first
     # read in metadata.py
-    with open(app.metadatapath, 'r') as f:
+    with open(app.metadatapath, 'r', encoding='utf8') as f:
         cur_content = f.read()
     metadata.write_txt(s, app)
     content = s.getvalue()