Drop the use of empty and . values for the form action attribute

[craigloftus]

Following Django, this commit drops the use of empty action attributes, inline with the HTML5 spec.

[codecov]

Codecov Report

Merging #2708 into master will not change coverage.
The diff coverage is n/a.

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  Hits        12983    12983           
  Misses       2539     2539

[solarissmoke]

For reference, here is the relevant section of the HTML5 specification:

7. Let action be the submitter element’s action.
8. If action is the empty string, let action be the document’s URL of the form document.

and:

The action and formaction content attributes, if specified, must have a value that is a valid non-empty URL potentially surrounded by spaces.

The action of an element is the value of the element’s formaction attribute, if the element is a Submit Button and has such an attribute, or the value of its form owner’s action attribute, if it has one, or else the empty string.