Add a Check for Updates button on the Upgrades page

[nbachiyski]

Currently the upgrade information from the API is cached for 12 hours inside WordPress. This causes problems for users, who either:

• want to upgrade right now and sometimes have to wait for hours.
• are international users, who see only the English version. The localized version are usually released at least several hours later.

One solution is to have a Check for updates now button, which gets the current version information from the API without checking the last time it was retrieved.

[ryan]

Other options are to always do a version check when visiting update-core.php or reduce the amount of time we wait between checks when visiting that page. We reduce the wait to one hour for plugins and themes when plugins.php or themes.php is visited.

[ryan]

(In [10583]) Check for new version when visiting Tools->Upgrade. fixes #9108