#3836 create accessiblity issue reporting template

[StephDriver]

new template for accessibility issue reporting.

using the a11y tag

based on bug report template

asks for missing alt-text not to be reported at present unless part of the alt text audit (to reduce likelihood of multiple reports - better to do this as one audit than accept these issue individually).

Closes #3836

[StephDriver]

future update of a11y template as issue #3839