Increase Channel Purpose length to 250, and add channel field length handling code to the slack importer (#4458)

* Increase Channel Purpose length to 250.

This commit increases the maxmimum length of the channel purpose field
to 250, including the database migration necessary.

It also adds a method to the Slack importer to check the lengths of
channel properties before importing, and truncate them if necessary.

Fixes #4168

* Fix database migration.

i18n/en.json

@@ -1671,6 +1671,22 @@
     "id": "api.slackimport.slack_parse_posts.error",
     "translation": "Error parsing some slack posts. Import may work anyway."
   },
+  {
+    "id": "api.slackimport.slack_sanitise_channel_properties.display_name_too_long.warn",
+    "translation": "Slack Importer: Channel {{.ChannelName}} has a display name which is too long. It will be truncated when imported."
+  },
+  {
+    "id": "api.slackimport.slack_sanitise_channel_properties.header_too_long.warn",
+    "translation": "Slack Importer: Channel {{.ChannelName}} has a header which is too long. It will be truncated when imported."
+  },
+  {
+    "id": "api.slackimport.slack_sanitise_channel_properties.name_too_long.warn",
+    "translation": "Slack Importer: Channel {{.ChannelName}} has a name which is too long. It will be truncated when imported."
+  },
+  {
+    "id": "api.slackimport.slack_sanitise_channel_properties.purpose_too_long.warn",
+    "translation": "Slack Importer: Channel {{.ChannelName}} has a purpose which is too long. It will be truncated when imported."
+  },
   {
     "id": "api.status.init.debug",
     "translation": "Initializing status api routes"