Gateway Payments availability condition for paid access

Version 1.0.5 build 2025120100

** Merged PR #2
** Fixed #10
** Fixed #6 (might be forced in the future)
** Fixed #5 (added "purchase description prepend string setting)
** Deprecated 'purchasedescription' string ID

Version 1.0.2 build 2021081602

** Fixed issue #1
** Added default configs for cost and currency
** Modified JS/YUI module to reflect introduced defaults.
** Added options to hide/disable payment button when "invalid".
** Added warnings for misconfigurations or when payments cannot be performed.

Version 1.0.0 build 2021081601

** Fixed most YUI form notices as indicated by Moodle's code checker results.

Version 1.0.0 build 2021081600

** Initial version