Prevent repeat dotfile edits and a more pleasant UX

Updates
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* Remove check for SSH key before copying. This shouldn't fail unless the previous keygen step fails, in which case this doesn't matter
* Check for existing dotfile changes before repeating them. Ensures idempotency of the script. Applies to Homebrew path priority change and rbenv init call.

UX
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* Add intro and pause at beginning (e.g. 'hit return to start')
* Clearer description of browser-opening step. Browser opening can surprise people. This gives you some context.
* Add trailing echo's to separate output
* Add pause and instructions for setting up dbs at boot

* Give a 'hit return to continue' to give users time to read
* Instruct them on how to set up DBs to start at boot