multipart: NextPart: bufio: buffer full

**Gorse version**
Gorse v0.4.0

**Describe the bug**
When you try to import a large CSV file containing items via the dashboard, in my case, the file was 39MB, it generates a "multipart: NextPart: bufio: buffer full" error.

**To Reproduce**
Setup Gorse via docker and try to import items via the web UI using a CSV file (my file is 39MB with over 80K rows).

**Expected behavior**
I expected that the items would be successfully imported and a message with the number of affected rows would be returned.

**Additional context**
My setup runs is:
OS - Ubuntu 18.04.4 LTS
CPUs - 8
Total RAM - 31.28GB
Docker version 20.10.14, build a224086
Server:
Containers: 5
Running: 5
Paused: 0
Stopped: 0
Images: 8
Server Version: 20.10.14
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Native Overlay Diff: true
userxattr: false