Set the default value to false
Adjust tests to expect false as the default
Update documentation to make new default clear

Add env_var and file as supported types for CI variables. Variables of
type file expose to users existing gitlab-runner behaviour - save
variable value into a temp file and set the path to this file in an ENV
var named after the variable key.

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.

Show masked switch for each variable
When toggled on, the variable value will be masked in runner logs
Show warning message if the switch is on but the value is not maskable

This reverts merge request !25476

Add a masked switch to variable rows
Copy some behavior from the protected switch