Only follow a blog when the message is directed at the bot

We should use the following syntax to start following a blog
```
@WordPressDotComBot follow https://myblog.com
```

Otherwise we might follow blogs inadvertently

Locate the bot's message handling and the code that starts following a blog. Check how messages are currently interpreted, then verify that only the shown @WordPressDotComBot follow URL syntax triggers a follow and that other messages do not. Add or update the relevant tests if the repository contains coverage for message handling.

Written by the indexing model from the issue text.