quotes for the plain text version + refactoring for #2957

[DenBond7]

This PR fixed quotes for the plain text version + refactoring for #2957

close #2961

How to test:

1. Use a web client to create a conversation
2. Create a message via recipientA
3. Use Plain mode (not HTML) and send a message to recipientB
4. Open a message via recipientB and reply via Plain mode
5. Open a message via recipientA and check quotes

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Tests (delete all except exactly one):

• Tests added or updated (Added only JUnit test. UI test is impossible as we can't get HTML from WebView via Espresso).

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities