This is a follow-up ticket of PLANET-2534. 

The goal is to implement an option for editors to create custom Tag pages.

Requirements:

a) In the tag page, add on the top a drop down page selector.

b) If the editor selects a page, then when the visitors goes to the front end url for that tag, they see the content of that page.

c) If the editor selects nothing, then (for now) the existing tag page is shown.

d) The url should remain as it is now.

e) Investigate if it is possible: When a visitor tries to go to the default url of the page, to be redirected to the tag page (so that we don't have two pages appearing in the site with exactly the same content).