Fix wiki encoding error creating event

What does this MR do?

Method add_bind changed in Rails 6 and now it includes a join. If any of the binding conditions is not in UTF-8 it fails (because of the join).

We have this problem with wikis. If the slug is not forced to UTF-8, when we create the event, the error incompatible character encodings: ASCII-8BIT and UTF-8 is raised.

In this MR, we're forcing the encoding of the wiki page title in order to avoid that problem.

More info about the debugging in #333464 (comment 600546102).

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #333464 (closed)