Improve the welcome and update notifications

[danielchalmers]

Problem

• Update notification click target. The update notification says "Select to see what's new", but selecting it opened Settings, not the release notes. This mismatch came in with Design and accessibility pass across all surfaces #86, which changed the copy.
• Update notification sound. It plays a sound, and it comes back on every launch until the update is installed. The startup notifications are already silent.
• Run at Windows startup. It's off by default and the first run never mentions it. A utility opened only by hotkey therefore stops working after the first restart, with nothing to say why.

Changes

• Update notification.
  • Selecting it opens the latest release page. TrayService.NotificationLink follows the existing NotificationAction pattern: the most recent notification's link, cleared by the next notification.
  • It no longer plays a sound.
  • The release page URL moves to UpdateService.LatestReleasePageUrl, shared with the Settings Download button.
• Welcome notification. While Run at Windows startup is off, it adds "To start Radial Actions when you sign in, select this and turn on Run at Windows startup." Selecting it already opens the General tab, which has that setting. ForStartup takes runsAtStartup, and the hotkey-problem notification is unchanged.

Validation

• dotnet build: 0 warnings. dotnet test: 606 passed. TrayNotificationTextTests cover:
  • the welcome with Run at Windows startup on and off, with and without a hotkey;
  • ForStartup passing the setting through.
• Ran a copy of the Debug build with a fresh settings file, without touching the installed app. To simulate a click, I posted the message the shell sends when a notification is selected (NIN_BALLOONUSERCLICK) to the tray icon's message window.
  • First run, update available (Debug reports 0.0.0). The welcome was accepted (its shown flag was saved). Selecting the latest notification opened "Release v0.7.0 · danielchalmers/RadialActions" in the browser, and Settings stayed closed.
  • Welcome as the latest notification (update check off). Selecting it opened Settings on the General tab, and no browser page opened.
• Not checked by eye or ear: how the notifications look on screen, and that the update notification is silent. The silence comes from passing sound: false, like the startup notifications.

Risks

• Two notifications on first launch. If an update is available, the welcome and update notifications appear seconds apart. A shell click doesn't say which notification was selected, so selecting the welcome after the update notification has shown opens the release page. This was already true for action-failure notifications, which work the same way.