Bug 28914: Fix wording in authentication forms

Bug 28914: Fix wording in authentication forms

In most authentication forms we see :
Fields "Login:" and "Password:" with a submit button "Log in".

In some places submit button contains "Login", which is confusing for translation.
It is not correct according to terminology https://wiki.koha-community.org/wiki/Terminology#L
Also in opac-user.pl ":" is missing, it generates new translation entries.

Test plan:
1) Log out if you are logged in
2) Go to staff interface
3) Check you see button "Log in"
4) Go to OPAC page /cgi-bin/koha/opac-user.pl
5) Check you see fields "Login:" and "Password:"

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>