Move from pico.css to mirageCSS (#1)

UI/UX changes: PicoCSS -> mirageCSS, rice-box.go removed and placed on .gitignore (since it is generated anyway), update templates.

Signed-off-by: Aoi K <koizumi.aoi@kyoko-project.wer.ee>

Co-authored-by: Aoi K <koizumi.aoi@kyoko-project.wer.ee>
Reviewed-on: https://git.mills.io/prologic/spyda/pulls/1
Co-authored-by: Aoi Koizumi (古泉 あおい) <novaburst@noreply@mills.io>
Co-committed-by: Aoi Koizumi (古泉 あおい) <novaburst@noreply@mills.io>

internal/templates/error.html

@@ -1,9 +1,9 @@
 {{define "content"}}
-  <article class="grid">
-    <div>
-      <hgroup>
-        <h2>{{ if .Error }}Error{{ else }}Success {{ end }}</h2>
-        <h3>{{ .Message }}</h3>
-      </hgroup>
-  </article>
+<article>
+  <div>
+    <hgroup>
+      <h2>{{ if .Error }}Error{{ else }}Success {{ end }}</h2>
+      <h3>{{ .Message }}</h3>
+    </hgroup>
+</article>
 {{end}}