Title and target not respected when adding a link to a table

[MaciekBaron]

When adding a link to a table, a user can specify the title and whether to open the link a new window or not:

This is currently not respected and silently ignored.

This is for two distinct reasons.

The title is ignored, because title is not passed in the Javascript (while target is):

wagtail-tinytableblock/src/wagtail_tinytableblock/static/wagtail_tinytableblock/js/tiny-table-block.js

Line 20 in 09e100d

valid_elements += ",a[href|target|rel]";

The target experiences the opposite problem. It is passed in the JavaScript, but not passed in the backend sanitisation (while title is):

wagtail-tinytableblock/src/wagtail_tinytableblock/utils.py

Line 26 in 09e100d

attributes["a"] = {"href", "rel", "title"}

Solution: Unify the attribute list so that all for attributes are included: href, rel, title and target.