Allow proxy config for message

[tanasegabriel]

Description

Allowing message to take in proxy configurations, similar to all other services.
I took the liberty of adding a new mixin, so that we have just one management point for this.

One thing that I explicitly did not do is allowing the top-level proxy to be inherited, e.g.:

diff --git a/src/App.vue b/src/App.vue
index 5860aef..0cf8376 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -66,7 +66,7 @@

         <div v-if="!offline">
           <!-- Optional messages -->
-          <Message :item="config.message" />
+          <Message :item="config.message" :proxy="config.proxy"/>

           <!-- Horizontal layout -->
           <div v-if="!vlayout || filter" class="columns is-multiline">

I would actually prefer it to be that way, but I think that will break a lot of configs where CORS is involved and an external endpoint is used.

In this case, people will have to add useCredentials: false to their message and/or alter their config and I'm afraid that constitutes as a breaking change,

Fixes # (issue)
Closes #939

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md).
• I've checked my modifications for any breaking changes, especially in the config.yml file