Various improvements to uid management in ldapadmin

Validation of uid field:
* use regexp for Java validation
* use the same regexp for javascript validation in public "new account" page
* configure the regexp as a new parameter in config module
* new parameter in config module to specify the uid validation regexp

Auto generation of uid field:
* add a parameter to disable automatic uid generation in public "new account" page
* use this parameter to allow edition of uid field in privateui "new user" page
* accept uid as an input in POST /users/ API
* new parameter in config module to disable auto generation of uid field

<!---
@huboard:{"order":598.95068359375,"custom_state":""}
-->

What about using Configuration bean instead ? (just a question, this also works for sure, just wondering if we could rely more on the Configuration object, but these variables might be very specific to these beans, so maybe it's not relevant)