Make sure currentlocale on edit is always set

[I-Valchev]

Fixes #1528

[I-Valchev]

@bobdenotter Also had to change _localeswitcher.html.twig since after this change currentlocale is always defined, hence the switcher is always shown. I'm a bit unsure about this though: a CT could exist with just one locale that is different to the default one, but that won't be visible in the admin panel with this latest change.

I can think of 3 possible solutions:

• always show the locale switcher (even if just 1 option exists)
• never show the locale switcher when just 1 option exists
• show the locale switcher when one option exists only if that option is different than the default site locale

What do you think?