Rename occurences of the file through the documentation.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

typo

[doc] Installation: moved wheel installation earlier in the process so we don't require build-essential

[doc] Arch Linux installation: modified the command for user creation so it works on both Debian and Arch

- missing "H" in "FUNKWHALE"
- be consistent on `$FUNKWHALE_VERSION` env var use

FW requires Python>=3.5 ; only Python 3.4 is packaged in Debian 8.

fix #300

Seems that the extensions are per-database.
Without that tweak, the install will fail at django migrations step, yelling for insuficient rights to install the accent extension itself.

Which will fail if the system does not contains this locale.