[Feature Request] Do not force start and end dates

[codebude]

Quoted from @DawidGardzinski in the original issue Feedback after 1-2 weeks of using Libris (#78).

It should be possible to leave the start date empty for "currently reading" books and the end date empty for "read" books. A user may know that they have finished a book without knowing exactly when, or know that they are currently reading it without remembering exactly when they started.

Books without these dates could simply be omitted from the timeline.

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Author: @DawidGardzinski
Original issue: #78