Add meta viewport for plain text

[joshliebe]

Task/Issue URL: https://app.asana.com/0/488551667048375/1207407556106243/f

Description

Fixes the issue reported in https://github.com/duckduckgo/Android/issues/4555

Steps to test this PR

• [ ] Visit http://textfiles.com/stories/13chil.txt (Or any other plain text link)
• [ ] Go to Settings > Accessibility
• [ ] Change the text size
• [ ] Verify that the text size of the file changes as expected

UI changes

[joshliebe]

• #4587 👈
• develop

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