samlauth 8.x-3.0-rc2

A new release because I made a dumb mistake. Another RC because reasons.

Changes since 3.0-RC1:

Back out cacheability of the responses to login/logout requests. (They have problems when used together with the 'destination' parameter, which won't be tackled before 3.0. 3.0-rc1 contained a 'requests_cache_http_secs' configuration setting, which has been backed out now.)

#3183789 by pvalappil, roderik: Add configuration for Error Redirect URL.

#3205250 by roderik, johnwebdev: better enable custom event subscribers to handle errors.

(Above two issues add two related configuration settings. These will likely be removed again during a 4.x rewrite that gives developers greater flexibility to override things by default... but they are helpful with the current code structure.)

#3204065: support Text (plain, long) fields for attribute mapping.

#3202505 by bricas: Fix validation error when trying to map attributes.

#3204065 by PCate, roderik: fix warning in field mappings screen.