feat: add possibility to hide upload button (#25)

I was asked to add the possibility to hide the upload button to avoid users confuse this tool for a file sharing app.
This is only on the front.

No breaking changes, by default the button is displayed.

Co-authored-by: Thibault Piron <thibault.a.piron@gmail.com>
Reviewed-on: #25
Reviewed-by: Julien Riou <jriou@monitoring@riou.xyz>
Co-authored-by: tapiron <tapiron@monitoring@riou.xyz>
Co-committed-by: tapiron <tapiron@monitoring@riou.xyz>

src/server/templates/index.html

@@ -31,9 +31,11 @@
                         value="delete-after-read" name="delete-after-read">
                     <label class="col-form-label col-form-label-sm" for="delete-after-read">Delete after read</label>
                 </div>
+                {{if .EnableUploadFileButton}}
                 <div class="col">
                     <input type="file" class="form-control" for="file" id="file" name="file" accept="text/plain" />
                 </div>
+                {{end}}
                 <div class="col">
                     <select class="form-select" aria-label="Expiration" id="expiration" name="expiration">
                         <option disabled>Expiration</option>