Allow uploading from plaintext EML files

[lwcorp]

Please allow choosing plaintext EML files as an alternative to a source server.

If the user should choose EML files, then:

1. View the source file/s to pull the date header
2. Upload the file/s while keeping the internal date, in imap_append fashion:

If [date] parameter is set, it will set the INTERNALDATE on the appended message. The parameter should be a date string that conforms to the rfc2060 specifications for a date_time value.

If you implement #5, I was think of something like https://jsfiddle.net/ztc610h2/:

<table>
<tr>
<td colspan=3>Source:
</tr>
<tr>
<td><input name="source" type="radio" value="server">External host: 
<td>Hostname: <input type="text" name="source_server"> Username: <input type="text" name="source_username"> Port: <input type="number" name="source_username"> <input name="source_ssl" type="checkbox">SSL
</tr>
<tr>
<td><input name="source" type="radio" value="local">Local files:
<td><input type="file" multiple accept=".eml">
</tr>
<tr>
![upload to imap](https://user-images.githubusercontent.com/1773306/113484739-992ec200-94b2-11eb-8da7-a754f834a20b.png)

<td colspan=3>Target:
</tr>
<tr>
<td>External host: 
<td>Hostname: <input type="text" name="target_server"> Username: <input type="text" name="target_username"> Port: <input type="number" name="target_username"> <input name="target_ssl" type="checkbox">SSL
</tr>
</table>
<br />
<input type="submit" value="Upload to target IMAP">