stop spammers from joining the group

[amiremohamadi]

currently we have a mechanism to remove the messages containing specific words:

WiseAssBot/src/bot.rs

Lines 203 to 204 in 98cb061

	for rule in &self.config.bot.rules {
	for word in &rule.contains {

we can extend this functionality to also prevent accounts with specific names from joining the group.
currently the configuration looks like this:

WiseAssBot/config-example.toml

Lines 10 to 13 in 98cb061

	[[bot.rules]]
	action = "block"
	contains = [
	"bad word",

to support this feature, a new action (e.g. block-join) can be introduced.