Fix occasional input freezing on pinned tabs

[ayoy]

Task/Issue URL: https://app.asana.com/0/0/1202668155654614/f

Description:
TL;DR: This patch removes unnecessary calls to showTabContent.

The fix for while background flashing introduced delaying a call to showTabContent
until any of webViewDidCommit, webViewDidFail or webViewDidReceiveChallenge is called
(with this code specifically).
This was done to ensure that tab content is only updated when the website actually starts loading
(or fails or requires authentication).
Only one of these events is needed to switch to the new tab content, so the merge publisher
should be limited to a single event. It appears that when navigating to websites behind DDG SSO,
such as Asana, webViewDidReceiveChallenge is called also after webViewDidCommit
(sometimes multiple times), which seems to occasionally be breaking keyboard input.

Steps to test this PR:
The scenario is reproducible with Grafana dashboard search:
https://grafana.duckduckgo.com/d/TDyfl_UWk/duckduckgo-home?orgId=1&search=open&query=

1. Open Grafana and pin the tab.
2. Restart the browser and go to the pinned Grafana tab.
3. Try typing in the search field; verify that it works.

In the current release app the input won't work until you click a mouse in the text field.

Testing checklist:

• Test with Release configuration
• Test proper deallocation of tabs
• Make sure committed submodule changes are desired

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Internal references:

Software Engineering Expectations
Technical Design Template
When ready for review, remember to post the PR in MM

[ayoy]

flatMap is replaced with map + switchToLatest (which is an equivalent of RxSwift's flatMapLatest, otherwise unavailable in Combine).

The idea here is to project a stream of tabContent events into a stream of webViewDid* streams, but only handle the last result stream. Adding this significantly reduces the number of unnecessary calls to sink.

To test it, you can remove switchToLatest and replace map with flatMap, and put a print statement inside the sink closure. Then observe 9-10 print outputs in the console when loading Grafana, while there are 4 calls when switchToLatest is used.

[samsymons]

I'm impressed at how clean this fix ended up being, nice work. I tested this on production and then on this build, it's perfect. 💯