fix(richobjectstrings): Add missing placeholder validation

[nickvergessen]

Summary

Some apps were using placeholders with spaces, @ and slashes. But the frontend only allowed [a-z\-_.0-9]+
https://github.com/nextcloud-libraries/nextcloud-vue/blob/master/src/components/NcRichText/NcRichText.vue#L396-L397
At the same time, using 0-9 only would break in PHP as that makes it not a string anymore but a number, so additionally we now require the placeholder to start with a-zA-Z

The recommended workaround is to not use "user ids" and other things directly, but instead using hardcoded strings like actor or "counting keys" like user-1, similar to how comments does it on it's activities:

• Actor on hardcoded subject:
  

  server/apps/comments/lib/Activity/Provider.php

  Line 110 in 096f893

  ->setRichSubject($this->l->t('{author} commented on {file}'), [

• Counting in user defined message:
  

  server/apps/comments/lib/Activity/Provider.php

  Line 162 in 096f893

  $message = str_replace('@"' . $mention['id'] . '"', '{mention' . $mentionCount . '}', $message);

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

LGTM

[come-nc]

Good apart from typo