Refactor and pull back single app functionality

[atc0005]

Overview

There are a lot of refactoring changes applied to better support sharing of common logic, clarify intent and in general cleanup the code. Admittedly this would have been better managed by separate smaller PRs but I was "on a roll" and didn't find a good stopping place until I was finished drafting test cases (separate PR).

New functionality has been added to the cmd/eslg app to assist with testdata generation.

Further refactoring is needed once I've had enough time to distance myself from recent changes and can look at the code with fresh eyes.

Added

• add Encode Randomly button to cmd/eslg app
• add QueryEscape button to cmd/eslg app
• add QueryEscape Randomly button to cmd/eslg app

Changed

• pull back functionality
  • cmd/usl/output
  • pull back ReadURLsFromInput
  • pull back ProcessInputURLs
  • pull back helper functions
• push down functionality
  • encoding support used by cmd/eslg
  • move private encodeInput func to internal/safelinks/EncodeInput
  • decoding support used by cmd/dslg
  • move private decodeInput func to internal/safelinks/DecodeInput
• rename Encode button to Encode All in cmd/eslg app
• cmd/eslg app no longer explicitly sets light theme
  • this causes the app to auto-detect dark/light theming like the cmd/dslg currently does
• require expected safelinks domain
  • require that the SafeLinksBaseDomain is present in url.URL values considered valid Safe Links URLs
• require host and scheme values for url.URL values considered valid URLs
• CleanURL now removes leading/trailing periods
  • the new trimEnclosingURLCharacters func handles this task and the existing angle brackets removal task
• update GetURL* funcs (and callers) to optionally match non-HTTPS prefixed URLs. This is primarily done to support generating (encoding) testdata files.
• refactor common behavior from newQueryEscapeButton and
  newEncodeButton funcs to separate newProcessInputButton helper func

Fixed

• clarify meaning of ErrNoURLsFound error
• debug logging tweaks and fixes (more needed)
• use ParsedURL values to package the original URL match pattern, and a parsed version in place of url.URL values
  • provide Trimmed method to return a "clean" version of the original URL pattern
• clean URLs before performing search/replace operations
• handle scenario where output == input (failed encoding or query escaping), ensure that empty input error has a trailing newline so that appended errors are displayed properly
• fix hasAcceptableURLPrefix helper func logic
• update URLs func to require basic validation steps to pass before considering url.Parse results a valid URL