Allow admins to remove branding and assurance messages.

This is a fixed version of the previous PR which was opened to an unwanted branch.

This PR is a response to general inability to focus due to the brightly coloured Prism message. It may sound a bit odd, but often I find myself unable to focus because Prism sends me messages like "It's like it never even happened.". That's the only reason I ever wrote this pull request. This is entirely configurable and is disabled by default. Also, if the configuration does not specifically disable the assurances and prefix (or if the config is missing those values), Prism will still send those parts of the message.

Included in this commit are changes to code so administrators may disable Prism's prefixes:
```java
return ChatColor.LIGHT_PURPLE + plugin_name+" // " + ChatColor.WHITE + msg;
```
changed to:
```java
return (prism.getConfig().getBoolean("prism.messages.branding", true) ? ChatColor.LIGHT_PURPLE + plugin_name + " // " : null) + ChatColor.WHITE + msg;
```

This also allows them to disable the messages of "assurance" (e.g. "Like a boss."):
```java
if(result.getChangesApplied() > 0 && prism.getConfig().getBoolean("prism.messages.assurances", true)){
    msg += ChatColor.GRAY + " It's like it never happened.";
```
changed to:
```java
if(result.getChangesApplied() > 0){
    msg += ChatColor.GRAY + " It's like it never happened.";
```

Also, this will change `--- Basic Usage ---` to `--- Prism: Basic Usage ---` if prefixes are disabled. If they're enabled, nothing will have changed.


Original commit message:
```
This will allow the administrator of a server to disable the prefix
"Prism //" if they want to, as the player and moderator know they're
using Prism and the brightly coloured message can be distracting when
focusing on a task such as rolling back grief. It will also allow the admin
to disable assurance messages such as "Like a boss.", "It's like it
never happened.", and "It's like it was always there." for the same
reasons as above - they can be distracting. This will enable server
admins to apply a more minimalisitic feel to Prism with messages such
as "67 reverted. 54 ignored." instead of "Prism // 67 reverted. 54
ignored. It's like it never happened."
```