accessibility fix for subscription form

[shrilakshmishastry]

What's the issue :
Currently, there are no ids or aria-labels in the public subscription forms. These are helpful for accessibility.

Link to issue

What's the solution:

Add for attribute to label tag and id attribute to input tag.

Label tag with for attribute takes care of handling accessibility for input tag. Refer this document for more info doc

Screenshot of proof:

[candideu]

Thank you @shrilakshmishastry !