Implements the brokers down splash page.

[rfornea]

-implements the "brokers down" splash page when the user tries to click "Upload a File" from the main path choice page -implements the "brokers down" splash page when the user tries to click "Start Upload" from the /upload-form page (in case they manually navigate to the /upload-form page in their address bar) -does not currently implement the "brokers down" splash page when the users tries to download since downloads could still work, but leaves a TODO for us to remember to add something later to handle the case for when the tangle is down as well.

-modifies the execObservableIfBackendAvailable method to call one observable if the backend is available and a different observable if it's not.
-modifies the goToUploadForm method in the navigation epic to call execObservableIfBackendAvailable and navigate to /upload-form if available or /brokers-down if not.

-fixes a few spelling errors

[rfornea]

It would appear that some files in this PR have been prettier'd that didn't get prettier'd in the last PR. You guys want me to make a separate PR with more prettier stuff so you don't have sift through it all to see the real changes? @AaronVasquez @EdmundMai