Use select2 for selecting administrators and heads of chairs

[chrisma]

On chair#edit (/chairs/:id/edit) and chair#new (/chairs/new) pages.
Use select2: https://github.com/hpi-swt2/wimi-portal/blob/master/Gemfile#L92

[tbjoern]

this is done partially already. the input fields have the class user-auto-complete which is select2 enabled. however select2 is only used for building the selection menu. select2 has a multi-select feature, which we should make use of.

[chrisma]

Nice! Also use select2 to edit chair members.

[chrisma]

Add i18n help text for secretariat role and admin role.

• Secretariat receives handed in time sheets per mail
• Admins are able to add people to chairs

[tbjoern]

also use sanitized paramerters in controllers