Fix scrolling and viewport bug on Safari

[huong-li-nguyen]

Description

• Fix bug on scrolling and viewport on Safari
• Remove MarkUpSafe dependency (as it was there only for pen-testing, but we don't use it anywhere anymore)
• Squeezed quick tidy of docstrings in this PR as well

The issues appeared on Safari because of two properties:

• overflow: hidden- for context see https://teamtreehouse.com/community/overflow-hidden-not-working-in-ios-safari
• calc(100%-Xpx) - had to replace percentage units with viewport height. I'll double-check with @pruthvip15 whether this is the right approach, but for now it fixes the issue and doesn't seem to affect the other browsers

@l0uden - this should fix the issue we've seen on Safari. Please double-check :) @nadijagraca - important to be aware of above such that we always double-check and fix before merge

Screenshot

Before (on Safari)

• Header was not visible
• Screen was being cut-off and not scrollable

After

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