Server update UI changes

Upgrade can now be done in two ways:

1. Using the original approach via pmm-update
2. Using the new pmm-server-upgrade approach

The original approach via pmm-update can be disabled via settings - env var "DISABLE_LEGACY_UPDATES=true"

UI logic:

1. The CheckUpdates response now includes two new fields
   1. 
      pmm_update_available - tells if pmm-update approach can be used
   2. server_upgrade_available - tells if the new pmm-server-upgrade is available
2. If none of these options is true, we cannot do an upgrade (even if it's available)
3. If server_upgrade_available is true, offer the new pmm-server-upgrade option
4. Otherwise offer the old pmm-update option
5. If server_upgrade_available is false, display a link to the docs which explain how to enable pmm-server-upgrade
6. Based on the option selected, properly set the "method" field when calling "StartUpdate" or "UpdateStatus"
7. PMM Server update logs
   • Make the logs container wider so it can fit longer messages
   • Maybe allow horizontal scrolling as well?
8. After the PMM Server update has finished
   • Keep the logs view opened
   • Show a message "Upgrade has finished", instead of saying it was successful. It can actually fail and the error is visible in the logs.