torconnect semantic and accessibility problems

Some issues with the "about:torconnect" page:

• The "Configure Connection…" button (#configureButton) opens the settings page in the connection pane, and places the focus in the search box for the settings. This disorienting when using a screen reader. I feel like "Connection Settings..." or "Connection Preferences..." would be clearer.
• The progress bar (#progressBar) has no semantics, so there is no indication of progress on a screen reader.
• The top bar which shows during connection assistance (#breadcrumbs) has no semantic structure. Its children are no focusable and cannot be activated with a keyboard.
• When there is an error, we might want to use an aria-live region to announce the problem. Currently all that happens is the button changes focus.
• The "view logs..." is not semantic and cannot be opened with a keyboard. Clicking "view logs..." does not take you to the logs, but to the connection settings. Fixed in !469 (merged).
• On the final step between "Trying one more time" and "Tor Browser still cannot connect" focus is lost (it was on the cancel button). It should restore to "Configure Connection..." button.
• The #countries selector is inside a #locationDropdown <form>, but its label #locationDropdownLabel is outside of it. The "Try a Bridge" button, which acts as the "submit" button for this <form> is also outisde of it. I'm not sure we really need the <form>.
• The "Frequently selected locations" and "Other locations" items in #countries are treated as disabled items, rather than as the accessible label for a role="group" element.

Some issues with the url bar:

• Tor connect tries to visually hide the url bar placeholder text: "Search with or enter address". But this still acts as the accessible name for the url bar when focused with a screen reader. No longer visually hide after !639 (merged).
• The "Not Connected" button in the url bar is not semantically a button and cannot be focused or activated with a keyboard. If you click it, it doesn't move focus into the "about:torconnect" page. Removed with #41608 (closed), it is not just a status in the title bar with no click behaviour.
• If you press Ctrl+K it will start a search in the URL bar, even before connecting. You can also press Up and Down to change the search engine (without any screen reader response). There's no indication that typing most addresses or searches will fail. No longer visually hide after !639 (merged).
• There's also a slight usability issue where if you start typing in the urlbar whilst a connection is on going (e.g. if you connect automatically and it hasn't finished yet) whatever you have typed so far will be lost once the connection is established.