Add validation to prevent update of a user or member to an invalid username (13)

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes #17347 and #14823

Description

The core of both reported issues is that whilst we can rely on ASP.NET Identity validating the user name characters, for updates we need to do it ourselves. So I've added that into the validation checks for both users and members.

I also fixed an annoyance with LastPass attempting to fill the user name and leading to accidental updates.

To Test:

• Allow for example a space to be a valid character by configuring:

  "Umbraco": {
    "CMS": {
      "Security": {
        "UsernameIsEmail": false,
        "AllowedUserNameCharacters": "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-._ @+\\",

• Create a member and a user with a space in the user name.
• Update configuration to remove space from being a valid character.
• Before the PR you'll find you can update members and users that have a space in their username.
• After the code in the PR is applied, you won't be able to.

[nikolajlauridsen]

Looks good, tests good