Issue title: Customize the registration page
 Issue body: It can be interesting the add options for the registration page like :
- Registration active Y/N (existing)
- Manual validation
- Personnalized fields (content stored for manual validation or sent via email)

 Comments: 
 Comment 0: I plan to remove the current registration system once I complete the billing plugin as it introduces a new registration system on the front and back end. 

I may introduce a separate plugin that solely focuses on free registration, but these changes are not possible with the way I currently have the registration system built.
 Comment 1: Any ETA on the working billing system?
 Comment 2: @BartManX Right now the entire project has been on hold, including VWI & the billing system, because of my classes and other jobs. I will try to work on this if I have any free time this summer, but my guess is that it will take at least 2 months if I do have the time to work on it consistently. Sorry for the delays.
{'source': 'GithubIssues', 'repo_name_with_owner': 'cdgco/VestaWebInterface', 'issue_id': 449694874, 'repo_stars': 108.0}