feat: Allow for replacing a single post's content

[wwerner]

In discourse, a topic can have several posts: The initial one and multiple replies.

Previously, this action could only add replies to the given topic.
Now it can also replace a post's content to allow for updates.

To do so, pass discourse_post_id to the action instead of discourse_topic_id.
I don't consider it super intuitive, but it allows for supporting it without breaking existing users.

It also no longer adds the Changelog <date> title:

• the action can be used for other purposes than changelogs
• it's redundant anyway: The topic for changelogs contains, well, changelogs and each post has a date that is visible anyway
• if you need it, you are free to add it to the content file to post in a previous build step

WDYT?