Werk #11658: Add icon for custom topics on update to 2.0

Component	User interface
Title	Add icon for custom topics on update to 2.0
Date	Nov 12, 2020
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

After updating to 2.0.0i1, custom topics that already existed in version 1.6 had no icon set.

Such topics will now get a standard icon while the config update.

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