This reverts commit 0cc0237.

Reverts #5361

There are some formatting issues that cause incorrectness in the HTML
structure, so @translatewiki cannot parse them correctly. This is the
cause of empty translatiion strings in #5274. Mainly, some elements are
missing their closing tags, e.g.:

```xml 
<string name="faq_answer_bug_reporting">&lt;p&gt;&lt;ol&gt;&lt;li&gt;From your %s app home screen, tap the menu in the top left corner.&lt;/li&gt;&lt;li&gt;Tap &lt;strong&gt;Share feedback&lt;/strong&gt;.&lt;/li&gt;&lt;li&gt;Follow the instructions to report the bug or share feedback.&lt;/li&gt;&lt;/p&gt;</string>
```
is missing an \</ol\> tag before the closing \</p\> tag.

I have looked at several of the other "broken" strings, and they have
similar issues. When @translatewiki encounters an unclosed tag, it
continues reading until it encounters a corresponding closing tag, in a
subsequent string, meaning any strings between the two points are
assumed to be part of a single string. This is obviously a serious bug
in translatewiki itself.

I think it is best to revert this PR and fix the issues first before
merging it again.

Co-authored-by: Ben Henning <[email protected]>

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## Explanation

Fixes #5357

This PR includes the changes previously included in #5361, with an
additional fix for missing closing html tags described in #5403:

```
faq_answer_app_language: Missing </strong> tag after the email address.
faq_answer_bug_reporting: Missing closing </ol> tag.
faq_answer_exploration_player: Missing closing </ul> tag.
faq_answer_update_app: Missing closing </ol> tag and closing </p> tag.
faq_answer_update_os: Missing closing </ol> tag and closing </p> tag.
```

This PR removes all CDATA declarations in the translatable strings.xml
file and instead escapes all necessary characters: < and > (& didn't
need to be escaped since no strings use that character at the moment).
This is needed because Translatewiki doesn't seem to extract the HTML
within the CDATA declaration correctly, so it may not be translated
(some existing strings were never translated, and per
#5274 the latest FAQ changes
aren't being processed correctly (leading to empty translation strings
being submitted).

This PR also introduces a regex check + test to ensure that CDATA isn't
used anymore in strings.

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