RxSwift / RxCocoa update to 5.1.1 / Deployment Target to 10.0

[John-Wolfe-Bose]

Got a warning from Apple that new app submissions will not be allowed to access UIWebView at all, and existing apps will not be able to submit new versions which access the UIWebView API starting in December 2020.

RxCocoa version 4.5 has a few extensions to UIWebView, and it seems like those extensions trigger the Apple check.

This pull request does several things:

• Update RxSwift / RxCocoa to version 5.1.1
• Add SwiftLint into the Podfile so it can be loaded locally
• Update SwiftLint build step for sample app to run first locally, then globally, then complain to user about missing SwiftLint
• Fixed SwiftLint orphaned_doc_comment warning
• Fix a BUNCH of RxSwift errors: 'Variable' replaced with 'BehaviorRelay' 'E' replaced with 'Element' Errors around DispatchTimeInterval initialization no longer assuming seconds (i.e., "5" changed to ".seconds(5)") 'next' deprecated
• Updated Intrepid Podspec to 0.16.0, deployment target to iOS 12.0
• Updated IntrepidTesting Podspec to 0.11.0, deployment target to iOS 12.0
• Ignore any .DS_Store files

[John-Wolfe-Bose]

@Nathaniel-Cox-Intrepid / @andrewdolce - how will you guys accept this PR? Will it be in a new branch rx_5.1.1 ? Or something different than that?

[John-Wolfe-Bose]

@andrewdolce - are you OK with these changes?

@andrewdolce / @Nathaniel-Cox-Intrepid - how do you want to land these changes? Into a new branch? Into a new release?

I am just circling around to make sure this PR does not get lost in the mists of time!