Rename duplicate searching to issue searching everywhere

[karenroldan]

What Does This PR Add/Change?

Duplicate searching is a bit narrow of a term, and it would be better to reference just "issue searching" or "searching for existing issues".

Since we're still pre-launch, let's just knock this all out at once and rename everything too, so that everything reads consistently and isn't confusing to future developers!

This changes include:

• Renaming the header, the page value in the url state, and all the copy/text re: issue searching.
• Renaming all the files, and folders to remove "duplicate"
• Rename all the component names, other function names, and the redux state nodes to remove "duplicate" from it.

Testing Instructions

• Make sure there are no compilation issues
• All tests are passing
• Manual regression test passes

Verify the following:

1. The page name is Search for Issues
2. Active page in the URL says search-for-issues
3. Track event page name is search-for-issues

Issues

Related to #
Closes #116