Support conversation description length from capabilities

[nickvergessen]

Hardcoded value of 500 in the past has to be replaced with a capability check for:

config => conversations => description-length (local) - The maximum length for conversation descriptions, currently 2000. Before this config was added the implicit limit was 500, since the existance of the feature capability room-description.

the 500 still need to be kept as a fall back for older server versions

• Changed in feat(conversations): Add new sample conversation "💡 Let's get started!" spreed#14124