Added custom override of hotkeys based on zuliprc file

[jamesdchu]

What does this PR do, and why?

This PR edits the keys.py file to enable custom hotkey mapping. This means that users can change the zuliprc file to get their desired hotkey mappings.

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes #
• [x ] Partially fixes issue Enable remapping hotkeys #1439
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• [x ] Adding automated tests for new behavior (or missing tests)
• [ x] Existing automated tests should already cover this (only a refactor of tested code)