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
should be set to empty fields, and set by the way of the updateLDAPadminMavenFilters method in https://github.com/georchestra/template/blob/master/build_support/GenerateConfig.groovy