Add of User Search, user creation and user card to user management page

[ericksantillanp]

What kind of change does this PR introduce?

• Bugfix
• New Feature
• Feature Improvement
• Refactoring
• Documentation
• Other, please describe:

Description:

This PR is WIP.
It implements a first version of an User Management page in the webapp, showing three main features:

• Gives the current number of users registered.
• Allows to create a user
• Allows to search a user by username or user_id

Missing features:

• Implement password verification for user creation
• Implement Delete user for the user card
• Implement Dashboard to see registrations.

This is how the UI currently looks like:

Checklist:

• I have read the CONTRIBUTING document.
• My changes don't require a change to the documentation, or if they do, I've added all required information.