Add retry button for failed feeds

[sedrubal]

Explain the Problem

There is no retry button for broken feeds and it seems that a failed feed won't be retried even if the server got fixed.

Steps to Reproduce

Explain what you did to encounter the issue

1. Add a feed
2. Hope that the feed will break (because the server is not accessible)
3. The feed has a yellow background and when you move the cursor on it, there is the title Update failed more than 50 times...
4. Fix the server providing the feed
5. I don't know what to do to get the feed working again

System Information

• News app version: 9.0.4
• ownCloud version: 9.1.4.2
• PHP version: 7.0.15
• Database and version: mariadb 10.0.29
• Browser and version: firefox 51.0.1
• Distribution and version: ubuntu 16.04.2

Contents of owncloud/data/owncloud.log

Contents of Browser Error Console