Profiles: Show the first sentence of forum reply on Activity tab, not the last one

[SergeyBiryukov]

Follow-up to #2222.

If a reply contains multiple quotes and multiple sentences, the activity entry displays the last sentence instead of the first one. That's because WPOrg_WP_Activity_Notifier::trim_text() uses a "greedy" selector.

See an ​example. My activity entry for that reply:

Posted a reply to Снова об XML-RPC, on the site Форумы поддержки:
Оставляю.

Correct entry would have been:

Posted a reply to Снова об XML-RPC, on the site Форумы поддержки:
Поменял на более нейтральный.

[coffee2code]

In 5116:

Profiles Activity Notifier: When trimming blockquotes, don't overzelously include original content between two separate blockquotes.

Props SergeyBiryukov.
Fixes #2492.