feat(webserver): password length and username can be configured

[andrewazores]

Fixes #341

• makes the webserver credential username configurable. Defaults to user as before, but this can be overridden if some need arises.
• makes the webserver credential password length configurable. This is always randomly generated, but the length was previously hardcoded to 24 characters. Now this can be configured, but continues to default to 24.
• expands the set of characters that can be generated for the password to include more of the ASCII symbols set. This increases password strength. Also simplifies the logic to just fill the password with any printable ASCII character, instead of trying to require at least one character from each of specific ranges first and then filling the remainder.

[mwangggg]

lgtm, just needs a rebase