Improve accessibility of tooltip help

[wpsmort]

Issue #2524

Proposed changes

This improves the accessibility of our tooltip help for users who use a screen reader. This PR makes three changes to the UI:

• It adds tabindex to the help icon so that you can tab to the help icon so that a screen reader can read the help text
• It adds the option name to the help text so that there is context to the help (which option the help is relevant to)
• It removes the colon from the end of all option labels (some have a colon some don't) so that our option labels are standardized and so that the option name in the help text can have a colon to add a pause when using screen readers

Types of changes

• Improves existing functionality (UI)
• Improves existing code

Checklist

• I've selected the appropriate branch (ie x.y rather than master).
• I've created an appropriate title, descriptive of what the PR does.
• Travis-ci runs with no errors.

Testing instructions

• Install a screen reader or enable the screen reader feature in your browser or OS
• Tab through all settings to verify you can tab from the help icon to the option input
• Verify that as you tab to the help icon, the screen reader should read the option name followed by the help text
• Verify that the colon has been removed from all option labels so that the standard for all options is no colon at the end of the label

Further comments

A note about removing the colon from labels: We've not had a standardized format for option labels in the past so some labels ended with a colon and some did not (older labels from pre v2.0 had colons).

This not only adds a standard label format for all labels but it also allows for the colon to be added in the help text which results in a pause between a screen reader reading the option name and the option text which is good for usability.

[arnaudbroes]

@wpsmort I made a small change where I added an H4 to make it look better for our regular users. Screen readers should ignore it. Let me know what you think.

[wpsmort]

This is tested and is good. The change made by Arnaud is good.

[EkoJR]

This looks good and approved