feat: add time zone setting for reminders (#1092)

Instead of naeveily checking for all reminders due in the next minute, we now check all reminders in all time zones in the next minutes. This essentially means checking for reminders due in the next 14 or past 12 hours. We then check for each user who would receive a reminder from that result if it is actually due in their time zone.

This should prevent issues where users would get the reminder in the time zone of their server, not in their own.

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1092
Co-authored-by: konrad <k@knt.li>
Co-committed-by: konrad <k@knt.li>

pkg/swagger/swagger.yaml

@@ -1284,6 +1284,10 @@ definitions:
         description: If enabled, the user will get an email for their overdue tasks
           each morning.
         type: boolean
+      timezone:
+        description: The user's time zone. Used to send task reminders in the time
+          zone of the user.
+        type: string
       week_start:
         description: The day when the week starts for this user. 0 = sunday, 1 = monday,
           etc.
@@ -6212,6 +6216,32 @@ paths:
       summary: Totp QR Code
       tags:
       - user
+  /user/timezones:
+    get:
+      consumes:
+      - application/json
+      description: Because available time zones depend on the system Vikunja is running
+        on, this endpoint returns a list of all valid time zones this particular Vikunja
+        instance can handle. The list of time zones is not sorted, you should sort
+        it on the client.
+      produces:
+      - application/json
+      responses:
+        "200":
+          description: All available time zones.
+          schema:
+            items:
+              type: string
+            type: array
+        "500":
+          description: Internal server error.
+          schema:
+            $ref: '#/definitions/models.Message'
+      security:
+      - JWTKeyAuth: []
+      summary: Get all available time zones on this vikunja instance
+      tags:
+      - user
   /user/token:
     post:
       consumes: