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.

webapp/components/edit_channel_purpose_modal.jsx

@@ -155,7 +155,7 @@ export default class EditChannelPurposeModal extends React.Component {
                         ref='purpose'
                         className='form-control no-resize'
                         rows='6'
-                        maxLength='128'
+                        maxLength='250'
                         defaultValue={this.props.channel.purpose}
                         onKeyDown={this.handleKeyDown}
                     />

webapp/components/new_channel_modal.jsx

@@ -283,7 +283,7 @@ class NewChannelModal extends React.Component {
                                         ref='channel_purpose'
                                         rows='4'
                                         placeholder={this.props.intl.formatMessage({id: 'channel_modal.purpose'})}
-                                        maxLength='128'
+                                        maxLength='250'
                                         value={this.props.channelData.purpose}
                                         onChange={this.handleChange}
                                         tabIndex='2'