Parent User implementation to inherit settings and permissions

[rkrenn]

this implementation provides the option to specify the parent user of a user.

a setting value can be either specified for the user or derived from the parent user ("inherited"). to inherit from parent, a checkbox can be ticked for each of the supported user settings:

UI:

locale
time zone
date format
decimal separator
theme
show tooltips

Decryption:

decrypt
decrypt from untrusted hosts

Tab&Module visibility:

enable inventory module
enable staff module
enable course module
enable trial module
enable inputfield module
enable proband module
enable massmail module
enable user module
list of visible inventory tabs
list of visible staff tabs
list of visible course tabs
list of visible trial tabs
list of visible proband tabs
list of visible inputfield tabs
list of visible user tabs
list of visible massmail tabs

Access Control:

user locked
inventory module permissions
staff module permissions
course module permissions
trial module permissions
proband module permissions
user module permissions
inputfield module permissions
massmail module permissions
inventory search permissions
staff search permissions
course search permissions
trial search permissions
proband search permissions
user search permissions
inputfield search permissions
massmail search permissions

when resolving a value, the chain of parents is traversed until it reaches an ancestor user that does no longer inherit. the UI precalculates a resulting value, which is indicated in tooltip texts or italic item labels.

closes #175
closes #176