I've just visited #electrum on IRC to get some advice. While restoring from a seed may make your wallet forget a transaction and allow you to resend it with a higher fee, abpa there told me that this sometimes doesn't work because the server will remind the client about the transaction if it's still in the server's mempool.
abpa suggested using
child-pays-for-parent to unstuck transactions, i.e. to send a transaction that spends the change output of the previous transaction with a higher fee to pay for both transactions. (Update:) Apparently, the Electrum developers have added GUI-support for CPFP in
Electrum 2.8.0 on 2017-03-09.
In general it is recommended to activate
replace-by-fee when using Electrum. This allows updating the fee after the transaction was broadcast if it turns out that the first estimate was insufficient. The setting can be found at
Tools > Preferences > Fees > Enable Replace-by-Fee.