Resolve issues with window.open / anchor target rewriting

[ikreymer]

Currently, there is no special handling for <a target=...> and window.open(..., target) values, which affect where the URL will be loaded.

In the case of window.open(), the url is rewritten without the mp_ modifier, causing it to open the top-frame in framed replay.
However, window.open(url, "_self") can result in the URL opening in the same window, so need to keep the mp_ modifier.

Also, opening in new window / tab can cause unexpected behavior if replaying in an embedded mode (eg. replayweb.page embed).

Additionally, _parent, and _top, can mess with the top-frame that contains the banner, which replayed pages should not access.

The ideal behavior is to navigate within the top iframe itself, which can be given a specified named (in pywb / wabac.js).
The name can be provided via wbinfo inject.

Plan to resolve as follows, for both anchor tag and window.open:

• If wbinfo.target_frame is provided, replace any of _parent, _top, _blank with that value, to ensure navigation happens with top frame.
• If target is any other value, keep as is.. (Unfortunately, no way to tell if a named 'browsing context' exists or not, so could still result in opening in a new window, or replacing an existing iframe?)
• Always rewrite with mp_, to load the replay. This may result in banner not being shown if somehow a new window is open due to named context.

Other option is to also replace named context with _self as well, thereby disallowing any other name, but that could break replay in some edge cases. (How often are named contexts used?)

For anchor tag rewriting, fix will likely involve pywb and wabac.js changes as well, while window.open() override can be handled here.