Use is_host from attendees table instead of post_author to determine who is a host

[psrpinto]

In #172 we made it so that the attendees table has an is_host field. However, this field is not yet being used everywhere to determine who is the host of an event.

We need to go over all places in the code where we use the post_author(or $event->author_id()) to tell who is the host of an event, and change it so that it uses is_host from the attendees table instead.

[psrpinto]

Related to #178

[akirk]

@amieiro this will solve the permissions problem you'll be facing in #200

[psrpinto]

I intended to take this issue, but I noticed it would need code that is being introduced in #200, so I think we should wait for #200 to be merged before tackling this one.

It could also be that #200 actually also fixes this issue. Once #200 is merged, we should check whether this issue is still relevant or not.

[trymebytes]

@psrpinto is it okay to close this?

[psrpinto]

I'm currently double-checking this, will close once I've confirmed it.

[psrpinto]

I've now confirmed this can be closed.