Add option to set a default timezone in the site (in the new version)

refs #671

cherry-pick 2bc7d13f78e32ad306022e17846df96dd9b21780 with adaptations
Created the missing translate strings in private messages, refs #297

cherry-pick 03719e307593e799f9de2c00f18b5ec2c698d290
Default site timezone to UTC

Partial cherry-pick of 9ad5ea2c65a757614c0b8017c7f85dbab2635d55
Modified the minimum length of user's username.

Partial cherry-pick of ce77beebfefa26deae9800e170f88065c2d197a2
Ignore blank timezones

If a user or site has a timezone as an empty string, for example, it should
be ignored.

cherry-pick 414552378747145d61fa61d1dcc5fd391db41804