Use first and last name fields for user

[ahmed-arb]

Description

This changes signup form to take first and last name instead of full name. The signup form will look like this:

Instead of

And if users want to change their name afterwards accounts settings page will be changed to take first and last name separately as well:

Additionally, sysadmin report, Download list of all users, will be changed to have first and last name.

[ziafazal]

@ahmed-arb looks good please squash commits while merging.