Forum RSS count of posts/discussions is not dependent on RSS setting

This is a minor usability issue.

When RSS is enabled a forum's settings includes settings to control the RSS feed coming out of the forum. The first setting is labelled "RSS feed for this activity" and has three options: "None", "Discussions" and "Posts". When this setting is set to none, no RSS feed is generated for the forum. The second RSS setting is the number of recent articles. This setting is active regardless of the state of the first setting. This second setting should be disabled when the first is set to "None".

Reproduction steps:

1. Log in as admin
2. In Settings > Site admin > Advanced features, enable RSS feeds
3. In Settings > Site admin > Plugins > Activity modules > Forum, enable RSS feeds
4. Navigate into a course
5. Create a new forum or edit an existing one
6. Set the setting labelled "RSS feed for this activity" to "None"
7. Attempt to change the setting labelled "Number of RSS recent articles"

Expected result: You should not be able to change "Number of RSS recent articles" while "RSS feed for this activity" is "None".

Actual result: You can.