Optimize tag invalidation

Problem/Motivation

At the moment each tag that needs invalidation is passed along with frontpage Url. If the number of tags is more than 100 (that is the default limit of urls to purge per 1 minute) then the purge request returns 429 response with following message:

{
   "code": 8,
   "message": "Too many CDN purge requests received. Please retry this request in 56 seconds.",
   "details": [
     {
       "@type": "stackpath.rpc.RetryInfo",
       "retryDelay": "56s"
     },
     {
       "@type": "stackpath.rpc.RequestInfo",
       "requestId": "75529166c9690116",
       "servingData": ""
     }
   ]
}

To avoid this the tags in purgeSelector can be concatinated.

Steps to reproduce

Create the page with a lot of paragraphs and/or block references, that will result in 100+ cache tags.

Proposed resolution

Use concatination in purgeSelector for selectorValue.

Remaining tasks

Create MR, test and merge.

User interface changes

None

API changes

None

Data model changes